1. Field of the Invention
The present invention relates to a zoom lens system to be used in a small and light-weight video camera or digital camera and the like. The zoom lens system has an aperture ratio of more than 1:2.8, a magnification ratio (zoom ratio) of approximately 3.0, and a simple structure which can be produced at low costs.
2. Description of the Prior Art
In recent years, along with further miniaturization and higher density of a small imaging device, such as a CCD, a zoom lens system which is utilized in video cameras, electronic still cameras and the like have also been required to be further miniaturized and to have higher performance. Furthermore, in a solid-state imaging device utilized in such video cameras and electronic still cameras, a color separation filter is provided in the close vicinity of a light-receiving surface of the solid-state imaging device. Due to this structure, the a bundle of light ryas which is obliquely incident on the light-receiving surface via the zoom lens system is interrupted by the color separation filter, which causes a decrease of peripheral illumination. Moreover, color irregularities occur due to misalignment of the color separation filter with respect to the pixels of the solid-state imaging device. Accordingly, in order to eliminate such drawbacks, an optical system in which the exit pupil is distant from the image plane is required. Due to this optical arrangement, a bundle of light rays can be incident on the light-receiving surface at an angle close to 90xc2x0. In other words, such an optical arrangement can achieve suitable telecentricity.
Furthermore, in recent compact zoom cameras, miniaturization thereof in a photographing state has been considered to be important; in addition, the compactness of the camera when the lens system is retracted and is being carried by a user is considered to be also important for further miniaturization. In other words, in such a zoom lens system, even when the lens system is retracted into the camera body, maintaining a slim camera body is required.
In order to achieve a slim camera body, thickness of each lens group which constitutes the zoom lens system has to be reduced; and in order to reduce the mechanical burden, it is necessary to reduce the traveling distance of each lens group upon zooming.
In a miniaturized zoom lens system of the prior art, a two-lens-group zoom lens system constituted by a negative first lens group and a positive second lens group is known. However, the exit pupil is relatively close to the image plane in many of such two-lens-group zoom lens systems, which is undesirable for a solid-state imaging device such as a CCD. Furthermore, when photographing an object at a closer distance, focusing needs to be carried out with the large-diameter first lens group, so that the focusing drive system undesirably becomes larger.
In order to overcome the above drawbacks, a three-lens-group zoom lens system which improves telecentricity has been proposed in Japanese Unexamined Patent Publication Nos. Hei-6-94996, Hei-10-39214, and Hei-11-194274. The three-lens-group zoom lens systems disclosed in these publications are constituted by a first lens group, a second lens group and a fixed or a movable positive lens group which is provided between the second lens group and an imaging device.
However, in the zoom lens system of Japanese Unexamined Patent Publication No. Hei-6-94996, only a small magnification ratio of approximately 2 is achieved, which does not satisfy the requirement for a higher magnification; while the telecentricity as the three-lens-group zoom lens system has been improved.
Furthermore, in the zoom lens system of Japanese Unexamined Patent Publication No. Hei-10-39214, a magnification ratio of approximately 3 is achieved with the same lens arrangement as that of Japanese Unexamined Patent Publication No. Hei-6-94996; however, the number of lens elements is large, and the overall length of the zoom lens system with respect to the focal length is long. Accordingly, the compactness of the camera when the lens barrel is in an accommodation position is sacrificed. Moreover, the zoom lens system of Japanese Unexamined Patent Publication No. Hei-10-39214 cannot be produced at low costs.
Furthermore, in the zoom lens system of Japanese Unexamined Patent Publication No. Hei-11-194274, the first lens group is constituted by two lens elements which include aspherical surfaces, and a small zoom lens system having a zoom ratio of 3 is achieved with 7 lens elements. However, since a plurality of lens elements having aspherical surfaces, which require a higher machining precision, are used, room of improvements still exist in regard to costs.
The present invention provides a zoom lens system to be used in a small and light-weight video camera or digital camera and the like. The zoom lens system has (i) an aperture ratio of more than 1:2.8 at the short focal length extremity, (ii) a magnification ratio of approximately 3.0, (iii) a half angle-of-view of at least 30xc2x0 at the short focal length extremity, (iv) an imaging capability adequate enough to cope with a high resolution imaging device, and (v) a simple structure which can be produced at low costs.
As an aspect of the present invention, there is provided a zoom lens system including a negative powered first lens group (hereinafter, the negative first lens group), a positive powered second lens group (hereinafter, the positive second lens group), and a positive powered third lens group (hereinafter, the positive third lens group), in this order from the object.
Upon zooming from the short focal length extremity to the long focal length extremity, the positive third lens group remains stationary, and the negative first lens group and the positive second lens group move along the optical axis of the zoom lens system.
The first lens group includes at least one positive lens element having a convex aspherical surface facing toward the image.
Furthermore, the zoom lens system satisfies the following conditions:
1.2 less than |f1/f2| less than 1.6xe2x80x83xe2x80x83(1) 
xe2x80x830.8 less than f3/f2 less than 1.3xe2x80x83xe2x80x83(2)
wherein
f1 designates the focal length of the negative first lens group;
f2 designates the focal length of the positive second lens group; and
f3 designates the focal length of the positive third lens group.
The negative first lens group includes a negative lens element having a concave surface facing toward the image, and a positive lens element having a convex surface facing toward the image, in this order from the object.
At least the image-side surface of the positive lens element is formed as an aspherical surface. The aspherical surface is formed so that the positive power increases as an increase of a distance from the optical axis, in comparison with the power of the paraxial spherical surface. Furthermore, the zoom lens system satisfies the following condition:
0.02 less than (xcex94a1xe2x88x92xcex94a2)/fw less than 0.08xe2x80x83xe2x80x83(3) 
wherein
xcex94a1 designates the amount of asphericity (including a case where xcex94a1=0) at the maximum effective radius of the aspherical surface in the case where the aspherical surface is provided on the object-side surface of the positive lens element of the negative first lens group;
xcex94a2 designates the amount of asphericity (xcex94a2xe2x89xa00) at the maximum effective radius of the aspherical surface in the case where the aspherical surface is provided on the image-side surface of the positive lens element of the negative first lens group; and
fw designates the focal length of the entire zoom lens system at the short focal length extremity.
Note that the amount of asphericity is defined as positive in the direction toward the image from the paraxial spherical surface.
The positive second lens group can include three lens elements, i.e., a biconvex positive lens element, and cemented lens elements having a positive lens element and a negative lens element, in this order from the object. The most image-side surface of the positive second lens group is formed as a strong divergent surface. Furthermore, the zoom lens system satisfies the following condition:
0.05 less than R2/TL less than 0.15xe2x80x83xe2x80x83(4) 
wherein
R2 designates the radius of curvature of the most image-side surface of the positive second lens group; and
TL designates the distance along the optical axis from the most object-side surface of the negative first lens group to the most image-side surface of the positive third lens group.
The positive third lens group can include a single biconvex positive lens element, and satisfies the following condition:
0.7 less than R3/f3 less than 1.3xe2x80x83xe2x80x83(5) 
wherein
R3 designates the radius of curvature of the object-side surface of the biconvex positive lens element.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-335437 (filed on Oct. 31, 2001) which is expressly incorporated herein in its entirety.